1. Field of the Invention
The present invention relates to an automotive exhaust system or a catalytic-reactor type apparatus and, more particularly, to an apparatus for supporting a ceramic honeycomb element, used as a catalyst carrier, on a casing in the above combustion apparatus.
2. Description of the Related Art
A number of combustion apparatuses or reactors, such as gasoline-powered automobile engines, gas turbines combustors, etc., employ the catalytic reaction system, in which a ceramic element having a honeycomb cell structure acts as a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst which contains a precious metal. It is inserted in a tubular support which in turn is inserted in a tubular casing. The honeycomb cell structure is made of ceramics such as cordierite, whereas the casing is made of metal, such as hastelloy or SUS-316, and the support is a ceramic fiber mat or the like. The support is required for the following reason.
Since the honeycomb cell structure is surrounded by the casing, gas can flow through the cells. As gas flows through the cells of the structure, the catalyst carrier and the casing are heated to temperatures as high as 700.degree. C. in the case of an automobile engine, and as high as 1,000.degree. C. in the case of a gas turbine combustor. The casing, which is made of metal, tends to expand when heated to high temperature. In contrast, the honeycomb element, which is made of ceramics, hardly changes in size, since ceramics has an coefficient of linear expansion as low as about 1.times.10.sup.-6. Although made of ceramic fibers, the support expands when heated. Hence, the support, as a whole, is soft and elastic enough to function as a shock absorber and also as a holder for the honeycomb cell structure. The support can absorb the compressive force of the expanding casing, while it is holding the honeycomb cell structure in place. Without the support, the structure, which is brittle, could be broken when the casing is heated temperatures as high as 700.degree. C. to 1,000.degree. C. and applies a compressive force to the structure. This is why the support is indispensable.
FIG. 9 is a cross-sectional view showing a prior art support which is a shock-aborbing mat 23 formed of ceramic fibers and wound around cylindrical honeycomb element 14. As can be seen in this figure, most of the cells of honeycomb element 14 are square in shape. Since most cells are square, element 14 is anisotropic in regard to its modulus of elasticity and its strength. More specifically, element 14 is about only ten times less strong against a force (especially a compressive force) acting on it in the direction given by .theta.=45.degree. (FIG. 10) than to a force acting on it in the direction given by .theta.=0.degree.. Thus, if a compressive force were to act on it in the direction given by .theta.=45.degree., honeycomb element 14 would break though it is strong enough against greater forces acting in other directions.
When shock-absorbing member 23 receives a compressive force, member 23, which is made of ceramic fibers, distributes this force uniformly on the circumference of honeycomb element 14. Thus, the force acts on element 14 equally in all directions. Consequently, honeycomb element 14 is liable to break from the force applied in the direction given by .theta.=45.degree. unless this force is extremely small. In other words, in the conventional honeycomb element support apparatus, the ceramic honeycomb element would be broken by a force smaller than the force corresponding to its average strength.